Patent GB-737972-A (GB)
Title: Photographic printing exposure control
IPC: G03B27/80
Filed: 1953-09-22
Abstract: 737,972. Controlling photographic printing exposures photo-electrically. DU PONT DE NEMOURS & CO., E. I. Sept. 22, 1953 [Sept. 23, 1952], No. 26110/53. Classes 97 (1), 98 (1) and 98 (2). [Also in Group XL (b)] During the photographic printing of variable contrast printing paper, the negative is scanned with a moving narrow beam of light, the radiation transmitted being converted to a photo-electric current from which are derived two signals, one controlling total exposure time as a function of negative density and the other, controlling the contrast of the printed paper as a function of negative contrast. In the embodiment to be described total exposure time is controlled in accordance with maximum negative density although this is not necessarily so while contrast is controlled in accordance with the difference of the logarithm of maximum and minimum light transmissions and by varying the proportion of blue and yellow printing lights. General arrangements.- Projection printer (Fig. 1). Light from a source 11 is directed by a mirror 3 on to a negative 4 through a projection lens 5 and on to the printing paper 6 which is carried on motordriven paper storage rolls 8. Part of the light is diverted by a beam splitter 20 to the photo-electric scanning apparatus comprising lenses 21, 22, mirror 23, a mask and a photo electric detector. The scanner is a conventional Nip Kow disc and the mask is shown in detail in Fig. 4. The apertures of the disc are so spaced that one aperture leaves the mask as the other enters it. Thus the total area of light incident on the cell is constant. The output of the cell is applied through a cathode follower to circuits which derive the maximum transmission and minimum density and from these factors calculate the total exposure of the negative and the proportion of that total exposure which is effected by means of blue light. In accordance with these calculations is determined the " dwell " periods of a disc 140 having three sections respectively opaque, blue and yellow. Total exposure control circuit (Fig. 9).-The voltage developed across photo-electric cell 29 is applied to the left-hand grid of a double triode valve 45, the right-hand grid being earthed. A signal representing the light transmission of the negatives is taken from the left-hand cathode while a reference voltage is derived from the right. The signal from the L.H. side is applied to a load resistor 71 and a peak-detecting germanium diode 72, which detects maximum transmission and applied through relay contacts (to be described) to a capacitor 73, the other side of which is connected to the left-hand cathode of a double triode 74. The reference voltage developed at the right-hand cathode of valve 45 charges capacitor 75. The left-hand side of double triode 74 with capacitors 73 and 75, variable resistors C1, C2 and resistor 76 constitute a " bootstrap " sweep circuit and its cathode is connected to the left-hand grid of a double triode comparator tube 78 in the lefthand anode circuit of which is the coil of relay K6. This signal is also applied to the light quality control circuit at the left-hand grid of a further comparator tube 128. Light quality control circuit.-This circuit is concerned with deriving a signal voltage equal to the difference between the logarithms of maximum and minimum light transmission. The E.M.F. derived from the left-hand triode of valve 45 is applied to the grid of the right-hand side of a triode 74 through a 10 Megohm resistor 101. The reference potential from the righthand side of valve 45 is applied to the cathode of valve 74. Tube 74 constitutes a logarithmic amplifier, the output of which is applied to a double triode 104 constituting an amplifier and cathode follower. A positive feed-back connection L29 prevents loss of high-frequency components. The cathode output of valve 104 is applied to a peak-to-peak detector 119 which thus generates across a condenser 120 a voltage proportional to the difference of the two logarithms. A proportion of the voltage, adjustable by resistor C3, is applied to the righthand grid of valve 128, the left-hand viewing the signal from the total exposure circuit. Operation (Figs. 9 and 9A).-The detailed construction of the exposure regulator is described below but it is to be understood that lines L25, L35 and L36 are connected to the regulator and that according to which is energized the opaque section, blue filter or yellow filter is positioned in the printing beam. In the starting position the opaque section of the regulator is positioned in the printing beam. Printing is initiated by the closing of foot treadle switch S10 which energizes relay K5 which locks, and a shutter motor M1 through contacts of switch S6. This motor is a standard printer component and releases the beam of lamp 11, then actuates switch S6 to close contacts S6b and stop itself. Motor M1 also closes momentarily switch S8 to cause a fresh length of printing paper to be brought into position, and S2, energizing relay K3 to energize printer lamp 11. During paper traverse switch S1 is closed by the paper traversing mechanism, energizing " scan " relay K2 which maintains the supply to " opaque " line L25, thus keeping the opaque portion of disc 25 in the light path. The scan relay K2 initiates scanning of the negative by completing the circuit from germanium diode 72 (V.S.) to the condenser 73 in the total exposure circuit and enables the reference voltage from the cathode of valve 45 to pass to the scanning condenser 75. Capacitor 73 charges to a voltage corresponding to maximum light transmission plus the zero signal voltage while capacitor 75 charges to the zero signal level. At the end of the scanning cycle when paper traverse is complete, switch S1 opens, deenergizing relay K2 and insulating condensers 75 and 73 from tube 45. The potential of capcitors 73 and 75 and of the left-hand cathode of tube 74 all rise at the same rate, the rate of rise being dependent on maximum light transmission and thus commencing the exposure cycle. The rise starts the left-hand triode of tube 74 functioning as a bootstrap sweep circuit, the output being applied to the grid of the left-hand section of comparator tube 78. When relay K2 opens, regulator disc 25 is shifted from " opaque " to its blue position by a circuit from line L12 through contacts of relay K3 (now closed) and relay K2 (now open) and the first position of relays K6 and K7 to " blue " line L35 positioning the blue filter in the exposure beam. The blue exposure period now commences. As described in connection with the light quality control circuit the potential appearing across condenser 120 represents the difference of the maximum and minimum densities of the negative, the range and zero level of the difference being controlled by variable resistors C3, C4. The signal derived from variable resistor C3 is applied to the righthand grid of amplitude comparator tube 128, the left-hand grid of which is, as previously stated, fed from the total exposure circuit, so that the signal applied to the left-hand grids of both of comparator tube 78 and 128 is the same. The potential of both the left-hand grids commences to rise in accordance with the left-hand cathode potential of tube 74. Eventually the current in both sections of the tube 128 is the same and relay K7 operates, energizing relay K1. This removes a short-circuit from resistor C2 to reduce the rate of rise of the potential of the cathode of tube 74. This takes account of the fact that the sensitivity of the printing paper to yellow light is less than that to blue. K7 also switches the supply from " blue " lead L35 of the exposure regulator to "yellow" lead L36 inserting the yellow filter in the printing beam. The potential of the left-hand grid of comparator 78 now continues to rise at a reduced rate until the two grids are at equal potential when relay K6 is operated, making its contacts K6b and K6c simultaneously. This transfers the supply from " yellow " line L36 to " opaque " line L25, stopping the exposure and momentarily energizing relay K4 to de-energize lamp relay K3, extinguishing the printing lamp and de-energizing K5 which then opens its locking circuit. Relays K3 and K5 again energize shutter motor M1 which closes the printing machine shutter and restores switches S2, S6, S8. Relay K3 earths capacitor 73 and short-circuits capacitor 120. The left-hand grid potentials of tubes 78 and 128 drop so that relays K6 and K7 de-energize and return to their first contacts. K4 remains operated for a period after de-energization because of condenser CX. This maintains contacts K4b open until motor capacitor M1c has discharged. Non-typical negative alarm.-Tube 90 is arranged to operate a warning 97 when a negative is regarded as non-typical. This is done by comparing half maximum transmission derived from germanium diode 72 and resistors 97, 92 with average transmission derived from the left-hand cathode of valve 45. These two quantities are applied respectively to the grid of valve 90. If they differ materially an alarm 97 connected between the anodes is operated. Construction of exposure regulator (Fig. 11). The regulator comprises three sections respectively opaque, blue filter and yellow filter. A central conducting ring 155 may be supplied by any one of lines L25, L35, L36 and a further connection to the conducting ring is made from solenoid 141. When the supply is applied to any of lines L25, L35 or L36 the solenoid is energized through the central conducting ring and by means of a pivot lever arrangement withdraws a detent 150 from a notch in a driving ring 143 and pushes against one of pins 144 to rotate the disc. Rotation ceases when the energized line is positioned in a cut-away of the conducting ring, thus de-energizing the solenoid and allowing detent 150 to enter the appropriate notch. A modified construction of exposure regulat